There is a need to detect remotely, i.e. from the surface, the presence of liquids in ground-holes. In a water-well, one of the common requirements is to determine at what depth the water level lies in the well, and whether there is a layer of gasoline, or other hydrocarbon liquid, floating on the water. If such a floating layer is detected, there is also a requirement to determine accurately the thickness or depth of the layer. Some hydrocarbon or other contaminating liquids sometimes encountered in groundholes are heavier than water, and might form a layer beneath the water. Again, it is a requirement that such layers be detected and their thickness measured.
It is well known to determine whether a detected liquid is, on the one hand, water or, on the other hand, a hydrocarbon, such as gasoline or oil. This determination is done by measuring the electrical conductivity of the detected liquid. Hydrocarbon liquids tend to have a higher electrical resistance than groundwater, to the extent that the difference in resistivity can be harnessed for the purpose of distinguishing hydrocarbons from groundwater, with adequate reliability and consistency, using simple electrical techniques. On the other hand, the resistivity of hydrocarbon liquids is so great that it is not possible, without using sophisticated equipment, to use resistivity as the measuring means for detecting the difference between the hydrocarbon liquids and air.
It is also well known to detect the difference between air and a liquid, whether water or a hydrocarbon liquid, by using the optical properties of the liquid.
The refractive index of a liquid is substantially different from the refractive index of air. The refractive indexes of the various liquids found in the ground are not well separated, so that it is not practical to identify a particular liquid remotely by measuring its refractive index. However, it is only the presence of liquid--any liquid--that needs to be detected optically. The nature or identity of the liquid can be determined by the measure of electrical conductivity, as described.